1. Field of the Invention
The present invention relates to dot-size correction or modification in half-tone images, for use in a process for producing colour-corrected or colour-modified colour separations to be used in the production of printing plates for printing reproductions of coloured originals, in particular of colour photographs or art-work.
2. Description of the prior art
Halftone colour separations used for producing printing plates for printing a multi-coloured reproduction usually need some correction before approval for printing is obtained. This correction may involve correction for hue, saturation and brightness value to obtain either a faithfull reproduction of an original or to introduce special colour effects in the reproduction.
One technique, known in the art, that can be used to obtain such corrections or modification is the so-called dry dot etching technique or dry etching technique.
This technique comprises the following steps.
First each area to be corrected or modified is isolated photographically by the photographic production of at least one so-called "drop-out" mask.
Secondly a corrected or modified halftone separation is made by performing in addition to the normally required exposure of a light-sensitive film to obtain an exact copy of a separation, an over-exposure both through the initial separation and through the photographically made drop-out mask.
The production of a drop-out wherein a certain area is set free i.e. isolated, can occur as follows.
The colour in an area to be corrected is e.g. identified through visual assessment by comparison of the area to be isolated with the differently coloured fields of a so-called colour chart.
On such a colour chart is indicated for each colour a particular corresponding combination of positive and/or negative halftone separations of he original image which needs to be used so that when an exposure is made through this combination of halftone positive and/or negative halftone separations in overlay onto a photographic silver halide emulsion film the area corresponding with the identified colour becomes a transparent region on said film.
For example, the colour "red" is isolated by photographing accurately in registrates the cyan positive separation with the separation with the magneta and yellow negative separations onto a mask film. One or more light-diffusing sheets are arranged in conjunction with the halftone colour separations to suppress the halftone dot structure in the mask. The thus exposed and developed mask film provides light shielding except for the red coloured areas to be modified or corrected in dot size.
Alternatively instead of visual assessement, to compare the area to be isolated with a colour chart one can measure by means of a densitometer the dot-size in a given area to be isolated on each of the halftone separations of the original image. These measurements are performed in registration on each of the halftone separations. The measured density values are expressed in percentage dot size.
A relationship exists between the percentage dot size measured on said halftone separations and the primary colour hue.
According to this relationship a corresponding overlay exposure combination of positive and/or negative halftone separations of said original image can be found for each colour, in other words for each particular combination of percentage dot values in each of the primary colour separations.
By means of this overlay exposure combination of halftone positive and/or negative separation of the original image a photographic mask can be produced in which a denselometrically measured area is reproduced as a clear part, in other words is isolated.
When each such mask is prepared, one has to identify the needed degree of colour correction by calculating the desired dot change.
A dot size change in an area of at least one halftone separation that has been isolated by means of photographic drop-out masking is carried out by performing in sequence the normally required exposure of a light-sensitive print film through the proper initial halftone separation (in order to obtain an exact copy) and a selected amount of over-exposure in registration and the initial halftone separation.
The above described procedure as well as the calculation of the over-exposure dose that is necessary for obtaining a certain percentage dot size change is extensively described in Research Disclosure July 1983, item 23135.
Further information relating to the dry dot etching technique can be found in the European patent applications no. 88200273.6 and no. 87201742.1.